In the present energy-saving era, compact self-ballasted fluorescent lamps that exhibit high luminous efficiency and long life are calling attentions as alternative light sources to incandescent lamps. Compact self-ballasted fluorescent lamps (hereafter simply referred to as “lamps”) have bent arc tubes. Roughly two design types of such lamps are available, namely, lamps having globes for covering arc tubes, and lamps not having globes for covering arc tubes. Hereafter, a lamp having a globe is referred to as a “globe-type lamp” and a lamp not having a globe is referred to as a “non globe-type lamp”.
The difference between a globe-type lamp and a non globe-type lamp lies not only in the presence of a globe, but also in the form of mercury enclosed within an arc tube. For a globe-type lamp, mercury is enclosed within its arc tube in the form of an alloy containing bismuth (Bi), indium (In), tin (Sn), and the like, i.e., in the form of an amalgam. For a non globe-type lamp, mercury is enclosed within its arc tube not in the form of an amalgam but in a single form.
For a globe-type lamp, too, mercury used to be enclosed within its arc tube in the single form. However, a globe-type lamp in which mercury is enclosed in the single form has the following problem. When such a globe-type lamp is lit, heat is trapped in its globe covering the arc tube, causing the temperature of the arc tube to increase excessively. Along with this, a mercury vapor pressure within the arc tube increases, thereby greatly degrading the luminous efficiency of the lamp. For a non globe-type lamp, mercury is still enclosed within its arc tube in the single form because the temperature of the arc tube does not increase much during lighting and so only subtle degradation of the luminous efficiency occurs.
In view of the above problem, a number of techniques for suppressing an increase of a mercury vapor pressure within an arc tube have been proposed as disclosed in Japanese published examined application Nos. H03-22016, H03-22017, H03-24018, and H03-24019. However, these techniques can be effective only for lamps of energy-saving type, such as a 9 W lamp that is an alternative to a 40 W incandescent lamp. For a 12 W lamp and a 22 W lamp that are alternatives to a 60 W incandescent lamp and a 100 W incandescent lamp, the temperature of an arc tube increases excessively during lighting, and accordingly, these techniques fail to effectively suppress the increase of the mercury vapor pressure.
With such a background, a technique that can be applied to a 12 W lamp and a 22 W lamp that are alternatives to a 60 W incandescent lamp and a 100 W incandescent lamp has been developed. The technique is to enclose mercury within an arc tube in the form of an amalgam as employed in a conventional lamp, instead of enclosing mercury within an arc tube in the single form. This technique has been found to prevent degradation of the luminous efficiency when using an amalgam selected from BiIn, BiPbSn, InPb, BiIn, and InPbSn. It should be noted here that this technique of using an amalgam is now predominantly employed for globe-type lamps.
As a result, for a 12 W lamp using a triple U-shape arc tube or a quad U-shape arc tube that is an alternative to a 60 W incandescent lamp, the luminous efficiency of 68 lm/W level has been achieved.
These various techniques described above have greatly improved the luminous efficiency of lamps. However, due to an amalgam enclosed within an arc tube for the purpose of suppressing degradation of the luminous efficiency, rising of the luminous flux at the lamp startup is delayed. To be more specific, mercury is absorbed by an amalgam when the lamp is turned off. This reduces the luminous flux at the lamp startup because a mercury vapor pressure at the lamp startup is relatively low as compared with a lamp in which mercury is enclosed within an arc tube in the single form.